During combustion of fuel and notably of gas oil in a diesel engine, the carbonaceous products tend to form carbonaceous particles, which will be designated in the following of the description under the expression of “soots”, which are said to be noxious both for the environment and for health. For a long time, there has been a search for techniques with which the emission of these soots may be reduced.
A satisfactory solution consists of introducing into the exhaust line a particle filter (or PF in the following of the text) which will block soots in its channels in order to let a gas escape without any soots. When a certain amount of accumulated soots in the PF is attained, the soots are burned in order to free the channels of the PF. This step for regenerating the PF is usually accomplished at greater temperatures than the temperature of the gas during normal operation of the engine, the soots usually burning in air at temperatures above 650° C.
In order to assist with regeneration of the PF, a catalyst is generally used which has the purpose of facilitating oxidation of the soots either directly or indirectly. By facilitating the oxidation of the soots is meant the fact of allowing their oxidation at a lower temperature so that this temperature is attained more frequently during normal operation of the engine. A portion of the soots may thus be continuously burned during the operation of the engine.
The catalyst also gives the possibility of lowering the temperature required for regenerating the PF so that the regeneration temperature is less than the combustion temperature of the soots in the absence of said catalyst. The catalyst also allows acceleration of the oxidation rate of the soots which allows a reduction in the required time for regenerating the PF.
Among fuel additives for assisting with regeneration of the PF, dispersions of rare earths, notably based on cerium are known for being efficient for regenerating the PF and contribute to the reduction of the self-ignition temperature of the soots or of the oxidation temperature.
Dispersions of iron compounds used as an additive of fuels may contribute to the reduction of this oxidation temperature of the soots.
It is thus sought to obtain dispersions having good dispersibility, high stability over time and further good compatibility in the medium into which they are introduced, and preferably sufficient catalytic activity at a relatively not very high concentration.